Fix input text color on dark GTK themes (fix mozilla/addons#657)

[futpib]

Fixes mozilla/addons#657.

This sets explicit color for input, textarea.

Before:

After:

[codecov-io]

Codecov Report

Merging #4531 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4531   +/-   ##
=======================================
  Coverage   95.34%   95.34%           
=======================================
  Files         198      198           
  Lines        4602     4602           
  Branches      944      944           
=======================================
  Hits         4388     4388           
  Misses        188      188           
  Partials       26       26

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[tofumatt]

Cool! Thanks for the fix 👍

[caitmuenster]

Thanks so much, @futpib! Your contribution has been added to our recognition wiki.

Would you be interested in creating an account on mozillians.org? I'd be happy to vouch for you!